The invention concerns a method of recovering the data clock signal for data in an incoming signal which comprises a data burst having a periodic preamble followed by a data sequence. Finally, the invention concerns a clock recovery module for performing the method, and a cordless telephone having such a module for recovering the signal clock for the incoming signal.
Transmission of data between a fixed part and a cordless telephone (portable part) takes place in the form of data sequences which are transmitted in time slots (TDMA--time division multiplex access). In DECT systems, a single data burst comprises a preamble, a sync word and a data sequence. The preamble, which is in the form of an alternating data flow and has a length corresponding to sixteen data bit periods, enables the receiver (the cordless telephone) to adjust the clock frequency and phase of the receiver to the transmitter. The sync word is used for clearly fixing the time of the arrival of the actual data sequence. Like the sync word, the data sequence is in the form of bits, which are GFSK-modulated in DECT systems.
When data in the data sequence transmitted by a fixed part upon a call are received by a cordless telephone, the preamble is used by the clock recovery module for finding the centre of the data pulses so that sampling can subsequently take place in an optimum manner. One bit quantization of the data pulses is usually less affected by noise in the centre. Therefore, it is frequently desired to place the sampling times centrally in a pulse.